DSOs (Digital Sampling Oscilloscopes) can be divided into two broad classifications: those that operate using ‘real time’ and those that operate using ‘equivalent time.’ A real time DSO takes consecutive samples at a rate high enough to meet Nyquist criteria for even ‘single shot’ events that contain frequency components up to the bandwidth of the ‘scope. And although a real time DSO will operate perfectly fine with repetitive inputs, there is no requirement that the input be repetitive to achieve full bandwidth. An equivalent time DSO cannot sample at the Nyquist rate, and relies upon the repetitive nature of the input to accumulate a replica of the input by sampling at a much lower rate, but at successively further locations along the input waveform, relative to a selected location identified by a trigger event.
Today, virtually all oscilloscopes are triggered, in contrast to the original technique of simply synchronizing a free-running sweep by adjusting its rate of repetition to produce a stable display. Indeed, in a modem DSO the notion of ‘horizontal sweep’ is almost an anachronism. The notion of ‘triggering’ has changed from when to start an instance of an analog time base to specifying a location in a time domain waveform that is understood as being a useful reference. That reference (the ‘trigger location’) can be selectively positioned within a displayed portion of that waveform, as reconstructed from digitized sample data stored in an acquisition memory. The idea is that the DSO is ‘triggered’ and that subsequently it will display a trace. The trigger event can occur before (what used to be called ‘delayed sweep’), after (so-called ‘negative time’), or during events represented by the displayed trace, depending upon such things as operator selection and the size of the acquisition memory. In any case, if there is no trigger event, then there is no displayed trace (unless an “AUTO” mode is in effect, wherein a trace is displayed at least every so often, even in the absence of a trigger event).
Correctly specifying the conditions under which the ‘scope triggers is thus a very important task. It is made more complicated when the signal being observed contains many different components, or types of behaviors. To assist the operator in getting the ‘scope to trigger as desired, it is common for the trigger event detection circuitry to be quite adjustable. Thus, it may include selection of AC or DC coupling, variable threshold detection (with hysteresis) for signal excursion, a refusal to re-trigger until a certain amount of time has past since the last trigger, and, of course, filters to either select or reject signal components based on frequency. The common term in use is ‘trigger rejection,’ although we should bear in mind that the choice of terminology is somewhat arbitrary, since what is ‘selected’ is whatever is not ‘rejected.’
Conventional DSOs still use analog trigger filter circuitry, just as in the olden days of analog ‘scopes. They are implemented with actual physical circuitry, and as such are limited in their range of operation by the dictates of circuit design and its compromises, they take space and generate heat, require adjustment, are susceptible of failure, and they cost money.